This invention relates generally to a telephone communications terminal, and, more specifically, to an electrical power conservation technique that is especially adapted for use therein.
In cases where a large number of individual telephone channels need to be provided between two geographical locations, it is common to multiplex a number of such channels on a single wide bandwidth circuit that extends between the locations. Multiplexing equipment is then provided at each location. This is done primarily to reduce the cost of providing transmission link for a large number of channels between the two locations.
An example is a loop carrier system provided between a large telephone company central office and a concentration of individual telephone subscribers located at least several miles away. A high bandwidth communications circuit is provided from the central office to the customer location, with a multiplexing terminal at each end. The use of a fiber optic transmission media between the terminals has an advantage over the use of a single electrical circuit in its greater bandwidth; that is, optical fiber systems can carry a much greater number of individual telephone channels. The channels are time multiplexed onto a single optical fiber transmission medium by providing repetitive time slots in sequence that are dedicated to each channel. Each terminal, therefore, provides for inserting signals from each channel being transmitted into its unique fiber optic time slot, and directs each received time slot to the correct individual telephone circuit. Each terminal includes a time slot interchanger that allows control of which end user telephone channels are assigned to which time slots in the fiber optic transmission system.
The usual time slot interchanger has a large random access memory ("RAM") with capacity enough to store a frame of data for the signals traveling in each direction through the terminal. A "frame" of data includes a digital sample of the signals traveling in each channel in one direction at a given instant of time. The data sample of each channel is assigned to a unique time slot within the frame. A number of separately addressable data storage locations are provided in the RAM sufficient to store at least two frames worth of signal samples. The order of the data samples read out of the memory is controllably different than that written into the memory, thus providing the function of rearranging (interchanging) the order of the time slots of the frame written into the memory and that read out of the memory.
Two techniques are utilized to bring about such an interchange. One technique is to write the time slot samples of the incoming signal into the RAM in the order received and then read them out randomly in the order desired for the outgoing frame. The second method is to randomly write the incoming time slot signal values into the RAM in the order desired for the outgoing signal and then read out the memory in sequence.
The RAM in such a system must have a number of separately addressable storage locations equal to or greater than the maximum channel capacity of the circuits connected to the interchanger in order to handle communication in one direction. If all of the channels are not being utilized in a particular system configuration, there are unused data storage locations in the RAM. There are two specific cases where this occurs. A first is where the channel capacity of the circuits on one side of the interchanger is much greater than that of the other side of the interchanger. In this case, there will be a large number of unused RAM storage locations at any time. A second case is in parallel implementations of time switches where multiple copies of data must be written for each channel. In this case, only one copy is read out at a time, making it unnecessary to access the RAM locations containing other copies. In each case, the number of storage locations provided in the RAM must be much greater than the number utilized at any one time. This requires providing power to unused RAM and creates an amount of heat that can be quite significant in a large communications terminal.
It is a primary object of the present invention to provide a technique for managing such a RAM in order to reduce its overall power consumption and amount of heat generated.